1. Field of the Invention
This invention relates to a charging member, a process cartridge and an electrophotographic apparatus.
2. Description of the Related Art
In electrophotographic apparatus, a roller-shaped charging member (hereinafter also “charging roller”) used in contact charging of an electrophotographic photosensitive member is kept in contact with the electrophotographic photosensitive member by pressing force of a spring or the like, and is so disposed as to be follow-up rotated with the latter. Hence, where the apparatus has been left to stand still over a long period of time, what is called permanent compression set (or “compression set”; hereinafter “C-set”) may come about at the part of contact of the charging roller with the electrophotographic photosensitive member. If the environment where the apparatus has been left to stand has high temperature and high humidity, the C-set tends to more come about.
In recent years, the electrophotographic apparatus are demanded to be much more high-speed, more high-quality for images and more high-durability, and it is sought to remedy the C-set. If the electrophotographic photosensitive member is charged with use of a charging roller having caused the C-set, it may become unable to maintain any uniform micro-discharge gap when the part standing come into C-set (hereinafter “C-set part”) passes through a discharge region, so that a difference in charging ability may unwantedly come between the C-set part and any non-C-set part. As the result, image density non-uniformity called horizontal black lines and/or horizontal white lines in the lengthwise direction (hereinafter “C-set images”) occurs at the position corresponding to the C-set part of the charging roller. The C-set images also more tend to occur when the voltage applied to the charging member is only direct-current voltage.
For such problems, Japanese Patent Application Laid-open No. H08-69148 discloses that an urethane foam provided with electrical conductivity by carbon black or the like is best as a material for charging assemblies that has a low hardness and also is less causative of compression set.